The present invention relates to local area networks (LANs) and, more particularly, to wireless LANs.
For a number of years, engineers have been investigating the possibility of having a LAN based on other than hard-wired transmission between the stations. Certain proposed systems have been based on the use of infrared transmission. Although useful in some applications, the communicating stations need to be within line-of-sight of each other, which may be disadvantageous in other applications. Accordingly, it has also been proposed to use radio transmission in such systems. Here, line-of-sight is not required. However, unlike the infrared case, multipath fading (hereinafter "fading") is a significant transmission impairment, particularly in indoor environments. Another significant problem is possible interference from other radio systems operating in the same frequency bands. These impairments can result in the loss of the transmitted information and, hereinafter, the word "loss" is used to refer to such information loss, whether arising from fading, interference or some combination of both.
Various techniques have been proposed to counter the fading and interference problems. For example, antenna diversity has been proposed to counteract fading. In addition, to counter both fading and interference, the use of direct sequence spread spectrum transmission or frequency hopping--possibly combined with some form of conventional channel coding--has been proposed. The present invention relates to the latter approach, which is described, for example, in A. A. M. Saleh and L. J. Cimini, Jr., "Indoor Radio Communications Using Time-Division Multiple Access with Cyclical Slow Frequency Hopping and Coding," IEEE Journal on Selected Areas in Communications, Vol. 7, No. 1, January 1989, pages 59-70, and in A. A. M. Saleh et al., "A TDMA Indoor Radio Communications System Using Cyclical Slow Frequency Hopping and Coding--Experimental Results and Implementation Issues," Conference Record of the IEEE Global Telecommunications Conference & Exhibition, Volume 3, November 28-Dec. 1, 1988, pages 1337-1342.
In particular, frequency hopping is a technique in which the information to be communicated is transmitted over a sequence of predefined "hopping" frequencies in a predefined order. The underlying theory of this approach is that loss and fading are not likely to occur over all frequencies at the same time. Thus, by "hopping" over several frequencies, at least some of the information will arrive at its intended destination intact. Advantageously, channel coding may be used in conjunction with the frequency hopping in such a way that the lost information can be recovered with high probability. Specifically, the information is encoded into a sequence of codewords, each codeword being made up of a sequence of signal points. Typically, the number of signal points in each codeword is the same as the number of hopping frequencies, and the signal points are rearranged for transmission in such a way that each signal point from each codeword is transmitted on a respective different hopping frequency. Thus even if certain of the signal points are lost, the inherent redundancy in the coding is such that, as noted above, the lost information can be recovered with high probability. In preferred systems embodying this approach, as described in the above-cited Saleh/Cimini paper, an interleaving approach may be used in which data signal points from a number of sequential codewords are transmitted on a single hopping frequency before transmission proceeds to the next hopping frequency. This advantageously reduces the rate at which the system needs to switch from one hopping frequency to another.